Our long term goal is to learn how the self-renewing asymmetric division of stem cells is achieved, a question crucial to the understanding of tissue development/repair, oncogenesis, immunodeficiency, and sterility. Our focus has been to address this question in Drosophila germline stem cells, initially by identifying these cells and revealing the self-renewing asymmetry of their division, followed by genetic dissection of the underlying mechanism. This leads to the finding of piwi, Yb, pumilio (pum), and arrest (aret) genes essential for germline stem cell renewal. Our working hypothesis is that the self- renewing ability of germ-line stem cells is induced by signaling from Yb/piwi/hh-expressing apical somatic cells. The somatic induction is achieved by controlling the asymmetric distribution of key proteins between the stem cell and its differentiated daughter, the cystoblast, which renders their specific fates. Such key proteins include PUM which regulates differential translation, BAM which promotes cystoblast differentiation and PIWI which is also expressed in the germline as a cell-autonomous promoter of stem cell division. Our specific aims are to explore the stem cell mechanism by systematically testing and expanding this hypothesis. With all the tools in place and experience in most of the following analyses, we propose to: 1. Analyze the biochemical role of PIWI as a nuclear factor in germline stem cell division. 2. Examine the developmental role of Yb in signaling germline stem cell division. 3. Analyze the biochemical role of YB in signaling cells. 4. Define regulatory relationship between genes involved in producing somatic signals. 5. Identify new genes interacting with piwi by genetic suppressor screens.